Final answer:
The aorta is often sheared at the aortic arch in deceleration injuries, which is the point of transition from the aortic arch to the descending thoracic aorta due to the combined effects of mobility and restraint leading to shearing forces.
Step-by-step explanation:
The aorta is the largest artery in the body and consists of several parts, including the ascending aorta, the aortic arch, and the descending aorta, which is further divided into the thoracic and abdominal aorta. The aorta is most often sheared in deceleration injuries at the point where the aortic arch transitions into the descending thoracic aorta, which is consistent with answer choice (a) Aortic arch. This area is vulnerable due to the mobility of the arch and the relative fixation of the descending aorta.
Deceleration injuries frequently occur in high-speed vehicular accidents when a rapid deceleration causes the heart and aorta to shift forward while the more fixed descending thoracic aorta is restrained, leading to a shearing force at the site of connection. The structure of the aorta, where the aortic arch descends and ends at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae, makes it susceptible to shearing due to sudden deceleration forces.